The present invention relates to remote seal assemblies. More specifically, the present invention relates a remote seal assembly including capillary weld extension with thermal isolation for coupling a process control instrument to a process fluid.
Some types of process control instruments, such as pressure transmitters, have a pressure sensor which is fluidically coupled to an isolation diaphragm by a capillary tube that is filled with a fill fluid. The isolation diaphragm comprises part of a subassembly called a “remote seal” or a “diaphragm seal”, which isolates the pressure sensor from corrosive process fluids being sensed. Pressure is transferred from the isolation diaphragm to the sensor through the fill fluid, which is substantially incompressible. The capillary or tube is typically flexible and may extend for several meters. The process medium exerts pressure on the isolation diaphragm which conveys the exerted pressure to the pressure sensor disposed in a transmitter housing separate or remote from the diaphragm.
Typically, the isolation diaphragm and any process-wetted parts of the remote seal are made from a corrosion resistive material, such that the process medium does not damage the subassembly. However, the housing of the remote seal may partially contact the process medium in some embodiments. Therefore, it is sometimes necessary that the housing of the remote seal either be protected from contact with the process medium or otherwise be corrosion resistive.
Within the process control industry, remote seal systems are typically used in extreme hot and cold applications to isolate the sensor from the process environment. The remote seal fulfills this function by thermally isolating the pressure transmitter from the process with an oil filled capillary system.
Unfortunately, the metallic, corrosion resistive construction of remote seal systems is known to outgas at high temperature. The term “outgas” refers to the emission of gases, such as hydrogen, from a solid material over time. In remote seal systems, when outgassing occurs, gas diffuses from the metal and enters the seal fill fluid. Diffusion of gases into the fill fluid is detrimental to the seal performance, particularly in vacuum applications.
Two sources of outgassing have been identified in remote seal systems. One source stems from gases dissolved in a bulk of the metal during fabrication. Another source involves gases dissolved in the welds used to join remote seal components. Typically, remote seals are formed from stainless steel, hastelloy, or other corrosion resistive materials, and sometimes contaminants or other gases are trapped in the bulk of the metal during the fabrication process. During welding processes typically used to join the remote seal components, gases become trapped in the weld material, creating a source for outgassing.
Steps can be taken to reduce outgassing in remote seal assemblies. For example, ultra-pure materials can be used to fabricate the remote seal components. However, even ultra-pure materials outgas when exposed to high temperatures over a period of time.
Another possible step is to perform a high temperature vacuum bake out of the entire remote seal system. The high temperature bakeout process reduces outgassing. Unfortunately, transferring this technique to remote seals in general is challenging due to the complex offering of remote seals available.
Moreover, while remote seal metal parts can be baked out as components to address gas source problems, this technique does not address the outgassing due to the weld joints. Vacuum bake out of the welds requires an assembly bake after construction but prior to use. An assembly bake of remote seal systems is extremely undesirable because of the multiple configurations offered for capillary size, length, and armor material. The assembly bake would also have to be part of the manufacturing process, adding significant process time and requiring multiple ovens worldwide for capacity reasons. Moreover, bakeout of an assembly with a polyvinyl chloride (PVC) armored capillary tube would be largely ineffective for high temperature devices, because the PVC could not withstand the high temperature bakeout required to eliminate outgassing from the corrosion resistive remote seal body.